Studies on Mechanical Properties of Brick Masonry

Abstract

Brick masonry, a composite of brick units bound together with mortar, is widely used for building construction in India. Burnt clay bricks are commonly used in construction of masonry structures since many years. But with growing environmental concern for conservation of natural resources and disposal of fly ash, bricks made with fly ash are emerging as a substitute to burnt clay bricks for construction of masonry structures. The behaviour of masonry structure isdependent on the properties of its constituents such as brick units and mortar separately and together as a unified mass. Brick properties vary largely from region to region as bricks are made with locally available raw materials with
inherent randomness. Therefore, the analysis and design of brick masonry structures considering the mean values of material properties may underestimate or overestimate the structural capacity. In order to design a safer structure it is necessary to take in to consideration the randomness and variability of the material properties of brick masonry. This requires mathematical description of
the variability in different material properties of brick masonry. The variability of mechanical properties related to steel and concrete is well researched, while the
same for brick masonry has not received proper attention. The lack of data has led to ignorance of uncertainty in brick masonry while analysing structures.
Under lateral loads, brick masonry is expected to undergo in-plane and out-of-plane forces. Resistance to out-of-plane forces in masonry structure is negligible and generally ignored in analysis and design. However, the in-plane forces which act parallel to the plane of wall is resisted by the bond between brick and mortar. Shear bond strength of masonry plays an important role in dealing with in-plane forces. The soaking of bricks prior to construction is very essential to achieve good shear bond strength. Bricks with higher initial rate of absorption must be pre-wetted prior to use in construction else they absorb more water from mortar inhibiting its hydration. But, the optimum duration of pre-wetting or the optimum moisture content of brick necessary to obtain higher shear bond strength
is not available in published literatures.
In present study, several experiments are carried out to determine mechanical properties such as initial rate of absorption, water absorption, dry density and
compressive strength of brick units, compressive strength of mortar and masonry prism and shear bond strength of masonry triplet. Higher order analyses such as
X-ray diffraction and field emission scanning electron microscopy are conducted to understand the morphological and microstructural differences in brick leading to
variation in its compressive strength. Three different types of failure patterns such as vertical splitting, diagonal shear failure and crushing are identified for masonry prism under axial compression. The variability in the mechanical properties of brick masonry and its
constituent materials is described using different probability distribution functions. Four two-parameter distribution functions namely normal, lognormal, gamma and Weibull distribution are chosen and their acceptability is evaluated using three goodness-of-fit tests such as Kolmogorov-Sminrov, Chi-square and log-likelihood test. All the distributions are found to be closely competing to fit the variability best. Lognormal is found to be common distribution function to best describe the variability for most of the mechanical properties studied. Weibull and gamma distributions are found to be most appropriate for other properties. However, in general, gamma distribution is found to be either the best or the next best distribution function to describe most of the mechanical properties studied.
Therefore, lognormal or gamma distribution is recommended as the distribution function that best describe the variability of properties of brick masonry and its
constituents. The morphological and microstructural analyses attributed the low and high
strength in brick samples to the absence of certain chemical compounds and variation in surface texture. The presence of compounds of silica, aluminium, calcium, iron oxide and magnesium is observed to be helpful for bricks in attaining higher compressive strength. Simple mathematical equations are proposed to estimate the compressive strength of brick unit and masonry prism. The equations can be used for both clay and fly ash bricks. The proposed equations are validated by comparing the
predicted value of the compressive strength with experimental value obtained from published literatures.
The optimum moisture content in bricks at the time of construction to obtain higher shear bond strength is experimentally determined. It is observed from the
failure pattern of triplets that shear bond failure depends on the strength of brick, mortar and their bond. IRA and moisture content of brick control the modes of
failure indirectly through shear bond strength